In recent years, it has been common for electronic devices, such as mobile phones, laptop computers, electronic notebooks, digital cameras, and video-game instruments, to have an audio function. These devices are desired to have a waterproof structure. However, a housing of an electronic device having an audio function is usually provided with openings located at positions corresponding to those of a sound emitter and a sound receiver such as a speaker, a microphone, and a buzzer, and sound needs to be transmitted through these openings, which makes it difficult to form a waterproof structure while ensuring the audio function. Hitherto, it has been attempted to ensure both the sound transmitting properties and the waterproof properties at an opening of a housing by covering the opening with a waterproof sound-transmitting membrane. The waterproof sound-transmitting membrane is a thin membrane made of a material that causes less disturbance in the transmission of sound. With the membrane disposed to the opening, it is possible to prevent entry of water into the interior of the housing while assuring good sound transmitting properties. A gas-permeable membrane that blocks entry of water and allows permeation of gases is suitable as the waterproof sound-transmitting membrane; more specifically, a gas-permeable membrane including a porous polytetrafluoroethylene (PTFE) membrane is suitable (see Patent Literature 1).
The intrinsic color of porous PTFE membranes is white. A waterproof sound-transmitting membrane is usually disposed in the interior of a housing so as to cover an opening. If the membrane is white, the membrane is conspicuous. Conspicuity of the membrane may be a defect in terms of the design quality of an electronic device, and is likely to stimulate the curiosity of a user and lead to damage to the membrane caused by stabbing with a writing instrument or the like. Therefore, a porous PTFE membrane that is black (black porous PTFE membrane) and that is thus less conspicuous when disposed on a housing has been demanded.
A black porous PTFE membrane is disclosed in Patent Literature 2. In addition, a porous PTFE membrane that is not black but colored gray or pink is described in Patent Literature 3.
A gas-permeable membrane including a porous PTFE membrane can be used not only as a waterproof sound-transmitting membrane but also as a waterproof and/or dustproof gas-permeable membrane that blocks entry of water and/or dust and that allows permeation of gases